Pneumatic tension control system

ABSTRACT

A pneumatic dancer web tension control apparatus for maintaining tension in a web being wound onto or unwound from a spool mounted on a spindle on a winding stand having a signal responsive brake for controlling the speed of the spool and a shiftably mounted sensing roll or dancer positioned to response to the tension of the web, the control system including a pneumatic brake control circuit having a compensating cam operatively connected to said sensing roll to respond to the movements of said sensing roll for generating a compensating signal for controlling said brake and a pneumatic dancer load circuit connected to respond to the movement of said sensing roll to maintain a steady and uniform tension on the web.

United States Patent [191 Abler n May 21, 1974 [5 PNEUMATIC TENSION CONTROL SYSTEM Primary Examiner-Leonard D. Christian [75] Inventor: Norman C. Abler, Menomonee Attorney Agent or firm-James Nlues Fall W' S 57 ABSTRACT [73] Asslgnee: Faustel Butler A pneumatic dancer web tension control apparatus for [22] Filed; Aug, 24, 1972 maintaining tension in a web being wound onto or unwound from a spool mounted on a spindle on a wind- [21] Appl 283489 ing stand having a signal responsive brake for controlling the speed of the spool and a shiftably mounted [52] us. Cl. 242/7543 n g ll r an r p siti ned to resp nse to the [51] Int. Cl B65h 25/22 s n of the the nt l syst m n luding a [58] Field of Search pn m r ke on rol circuit having a compensat- 242/75 43 75 53; 83/75 ing cam operatively connected to said sensing roll to 7 respond to the movements of said sensing roll for gen- [56] Reference Cit d erating a compensating signal for controlling said UNITED STATES PATENTS brake and a pneumatic dancer load circuit connected to respond to the movement of said sensing roll to 5333233 25132? 355123222:1111j:13:11333331133253??? nnninnnn n steady and nnifnnn nnnnnn on the 18 Claims, 5 Drawing Figures E/fl M8 [50 K )4 l5? M3 52 7! is new 7 "Z2 75 n e A Q 7? I 7/ i [7 73 PMENT ED W21 m4 smwma lffddId/l/A? 1 PNEUMATIC TENSION CONTROL SYSTEM BACKGROUND OF THE INVENTION Machinery manufacturers have striven to develop an all-pneumatic system that would regulate tension in a web accurately through a wide range of operating conditions in a stable and otherwise reliable manner. An apparatus as disclosed in US. Pat. No. 3,630,463, has been developed in an effort to achieve this result. However, this apparatus although satisfactory for some purposes does not have the degree of sophistication required to compensate for all of the various conditions encountered in controlling web tension. More recently, efforts have been centered on' developing electronic tension controls because of the highly developed automatic control theory which can be applied to many varied processes. The electronic techniques used in developing the systems require the incorporation of a large number of electronic elements in order to compensate for various conditions.

SUMMARY OF THE INVENTION The improved pneumatic dancer system of the present invention provides very steady and uniform web tension through a wide range of dancer excursions. The dancer system is capable of maintaining constant tension during high frequency small amplitude oscillations sometimes introduced during steady state running as well as for low frequency high amplitude excursions encountered during web splices, roll changes and the like. The system actually compensates for variations in WR which occur in roll inertia as diameters change with buildup on winders or builddown on unwind devices. Automatic gain compensation is incorporated into the system to compensate for changing the steady state system sensitivity comparable to the torque versus tension relationship changes which occur with roll buildup or builddown.

DRAWINGS FIG. 1 is a schematic diagram of the brake control circuit and the dancer load circuit for a web tension system having a dancer roll acting in an upward direction;

FIG. 2 is a schematic diagram of the brake control circuit and the dancer load circuit for an alternate upwardly acting dancer roll arrangement;

FIG. 3 is a schematic diagram of a brake control circuit and dnacer load circuit for a downwardly acting dancer roll assembly;

FIG. 4 is a schematic diagram of a brake control circuit and a dancer load circuit for an alternate form of downwardly acting dancer roll assembly; and

FIG. 5 is a side view of the compensating cam for the brake control circuit.

DESCRIPTION OF THE INVENTION The Web Unwind Stand A typical web unwind stand of the type in which the web tension control apparatus of the present invention can be used is shown in the Mistele US. Pat. No. 3,630,463. As seen therein, the unwind stand generally includes a pair of spaced vertical base members having a journal mounted on the upper edge of each of the base members for supporting a pivot shaft for a turret. The turret includes a pair of arms which are fixedly kll mounted on the pivot shaft and a pair of spindles are mounted in a parallel relation at the ends of the arms. This type of unwind stand is generally well known. Web supply rolls 12 are normally mounted on the spindle for unwinding. In FIG. 1 of the present application, only one of the spindles 10 is shown with the roll 12 mounted on the spindle 10.

The speed of rotation of the spindle 10 is controlled by means of a friction brake 16 so as to retard the rotation of the spindle 10 as well as the tension of a web 15. The brake 16 may comprise any suitable commercially available pneumatic type brake. The brake 16 is controlled by means of a brake control circuit 17 to provide a constant tension on the web 15 as it is unwound from the roll 12. The web 15 is unwound from the roll 12 and strung across guide rolls 22 and 24 which are closely spaced to permit a loop 23 to be formed in the web 15 by a dancer roll 18. Although the web 15 is shown as being unwound from the roll, the apparatus has similar application in a windup roll arrangement, in which a pneumatic clutch would be applied in place of the pneumatic brake.

The dancer roll 18 is supported at either end by pivot arms 20 connected to a common pivot shaft 14 which is pivotably mounted on a pair of fixed pivot arms 19. .The tension in the loop 23 formed in the web 15 is maintained by means of an air cylinder 52 having a piston rod 53 operatively connected to the pivot arms 20. The response of the air cylinder 52 to variations in tension in the web 15 is controlled by means of a dancer load circuit 50 as described hereinafter.

The Dancer Load Circuit The dancer load circuit 50 includes the air cylinder 52, an accumulator 54, a check valve 56, a pressure regulator 58, and a rate compensated differential pressure relief valve 62. As seen in FIG. I, the air cylinder 52 includes a piston 64 operatively connected by means of piston rod 53 to one of the support arms 20 for the dancer roll 18. The accumulator 54 is connected to the air cylinder 52 through a pressure line 66. Air under pressure is delivered to the air cylinder 52 through the line 67 to the pressure regulator 58 through the line 68 to check valve 56 and through line 69 to line 66. Pressure variations normally caused by high frequency low amplitude dancer excursions are filtered out by means of the accumulator 54.

Pressure variations normally caused by high amplitude dancer excursions are filtered out by means of the differential pressure relief valve 62. The relief valve 62 is connected to the air pressure line 68 by line 70 and to the line 69 by a line 71. It should be noted that the relief valve 62 is connected across the check valve 56 to relieve high pressure pulses in line 69, produced by high amplitude excursions of the dancer roll 18.

In this regard, the relief valve 62 includes a housing 72 having a piston rod 74 supported for axial move ment in the housing. A valve 75 is provided on the end of the piston rod 74 for controlling flow through a relief port 76 to atmosphere through vent 73. The piston rod 74 is secured to a diaphragm 78 which separates the housing 72 into an input pressure chamber 80 and an output pressure chamber 82. The piston rod 74 is biased to a closed position by means of a spring 77.

Under static operating conditions the pressure across the diaphragm 78 will be equal due to the line connections 70 and 71 to the input lines 68 and 69, respectively. When a pressure surge or impulse is produced in line 69, the pulse is prevented from entering line 68 and is conducted into chamber 82 through line 71. Since the pressure in both chambers 80 and 82 is equal to line pressure, the pressure pulse has to exceed only the bias of the spring 77 in order to move the piston rod 74 away from port 76 to vent the chamber 82 to atmosphere. The operating pressure of the relief valve 62 automatically compensates for line pressure variations because of the line connection 71 between the input line 68 and the chamber 80.

Means are provided for returning the pressure across diaphragm 78 to a normal condition very rapidly. Such means is in the form of a needle valve 60 which is connected across the check valve 56 by a line 84. Any small or minute variation in pressure across the check valve 56 will be transferred to both sides of the diaphragm through the needle valve 60 without any movement of the relief valve.

The Brake Signal Circuit The brake signal circuit 17 generally includes an infinitely variable ratio relay 92 connected to the air supply by line 94 and to the brake 16 by a line 96. The infinitely variable ratio relay 92 is controlled by means of a brake pressure regulating valve 98 connected to the air supply by a line 100 and to the ratio relay 92 by a line 102.

In accordance with one aspect of this invention, the brake signal pressure regulating valve 98 is controlled by means of a compensating cam 104 mounted on the end of the pivot arm 14 for the dancer roll 18 and mechanically coupled to the pressure regulating valve 98 by means of a cam follower 106. The angle of rise of the cam periphery 105 as the diameter of the roll decreases, FIG. 5, has been designed by writing the equation for constant gain and integrating the equation with respect to dancer travel, in order to evolve an expression for the cam shape for constant gain. This is a cam whose angle of rise (1) equals K (log D log d) where d) equals the cam angle, D the diameter of a full roll or spool, and d is the instantaneous diameter of the roll or spool. K is a constant of proportionality which depends on the relationship of dancer roll limits, cam rise and cam angle limits. For a given value of (I), the cam rise equals K where K is a constant of proportionality tions in roll diameter by providing through the cam periphery 105 a variation in signal from the pressure regulating valve 98 to the infinitely variable ratio relay 92. The relay 92 includes a second brake pressure regulating valve 108 connected across the lines 94 and 96. Any response to variations in pressure in the pressure regulating valve 98 are immediately compensated for by the relay 92 and transferred to the second regulating valve 108.

In this regard the infinitely variable ratio relay 'includes an air pressure regulator 108 connected to a lever 157 by means of a spring 150, and an air cylinder 155 including a piston 156 which is connected to the lever 157 by means of a connecting rod 154. The amount of change in variation in force of spring 150 produced by movements of piston 156 is controlled by means of a fulcrum 151 which is slidably attached to the lever 157. The fulcrum 151 can be positioned anywhere between the connections for the spring 150 and The brake signal circuit 17 compensates for varia- I the rod 154 on the lever 157 changing the relationship of segment 152 on lever 157 to segment 153 on lever 157.

In operation, air pressure introduced to the cylinder 155 from line 102 exerts a force on the piston 156. This force is transmitted to lever 157 by connecting rod 154. The force exerted by connecting rod 154 on lever 157 causes a force to be exerted on spring 150. The force exerted at spring 150 is proportional to the ratio of segment 153 of lever 157 to segment 152 of lever 157. The force exerted on spring 150 results in a proportionate output pressure on line 94 of regulator 108.

The net result of this multiplication or divison is that the regulator 98 can always operate through its full pressure range while unwinding from full roll diameter to core diameter, while the brake pressure can operate through any range to suit web tension required by proper positioning of the fulcrum 151. This adapts the compensating cam 106 to ideal conditions for any tension required.

FIG. 2

In the embodiment of the invention shown in FIG. 2, a web roll 12 is shown mounted on a spindle 10, as described above, with the web 15 threaded around guide rolls 22, the dancer roll 18 and the guide roll 24. However, the dancer roll 18 is supported by means of a pair of chains 116 mounted on sprockets 118 and 120. The sprockets 118 are mounted on a shaft 122 which is journaled for rotation in a supporting frame. The sprockets are mounted on a shaft 124 which is also journaled for rotation on a supporting frame. The tension in the web loop 23 around the dancer roll 18 is controlled by means of the brake control circuit 17 which is controlled by means of a compensating cam 104 mounted on the end of shaft 124. The operation of the brake control circuit 17 in this embodiment of the invention is essentially the same as described in connection with FIG. 1. Whenever the shaft 124 rotates, the control pressure of the pressure regulating valve 98 is varied due to the change in position of the cam 104.

Dancer roll excursions are controlled by means of the dancer load circuit 50 which includes an accumulator 54 and a rate compensated differential pressure regulator 62. Dancer excursions are sensed by means of a piston and cylinder assembly 52 and transferred through the pneumatic pressure line 66 to the accumulator 54 and to the differential pressure regulator 62.

In this regard, the piston and cylinder assembly 52 includes a piston 64 having a piston rod 53 connected to a gear segment 126 mounted on a pivot shaft 128. The gear segment 126 is connected to a drive gear 130 mounted on the end of the shaft 124. Any movement of the dancer roll 18 will produce a corresponding movement of the drive gear 130 and gear segment 126 which is sensed by the piston 64 producing a pressure variation in the line 66. The operation of the dancer load circuit is essentially the same as described above.

FIG. 3

In the embodiment of the invention shown in FIG. 3, the web roll 12 is mounted on a spindle 10 with the web 15 of the roll 12 threaded over guide rolls 22, around dancer roll 18 and over a guide roll 24. The tension in the loop 23 of the web is sensed by the dancer roll 18 which is supported for vertical movement on chains 116 which are mounted on sprockets 118 and 120. The sprockets 118 are mounted on shaft 122 and the sprockets 120 are mounted on shaft 124. However, in this embodiment of the invention, the dancer roll 18 acts in a vertically downward direction on the web loop and dancer excursions are compensated for in both upward and downward directions.

Dancer excursions in the upward direction are suppressed by means of a dancer load circuit 50a which is identical to load circuit 50 in FIG. 1. The dancer load circuit 50a includes an accumulator 54a and a differential pressure relief valve 62a connected to a piston and cylinder assembly 52a by pressure line 66a. The piston and cylinder 520 includes a piston 64a having a piston rod 53a connected to a gear segment 140 pivotably mounted on pm 141 and operatively connected to a drive gear 142 on the end of shaft 124. Pressure pulses produced in assembly 52a will be conducted through line 66a to either the accumulator 54a or the differential pressure relief valve 62a as described above.

Dancer excursions in the downward direction are controlled by means of a dancer load circuit 50b which is also identical to the load circuit 50 in FIG. -1. The dancer load circuit 50b includes an accumulator 54b and a differential pressure relief valve 62b connected to a piston and cylinder assembly 52b by means of an air line 66b. The piston and cylinder assembly 52b includes a piston 64b and a piston rod 53b connected to the other side of the gear segment 140. Any sudden downward movement of the dancer rod 18 will produce a pressure pulse in line 66b which is suppressed by either the accumulator 54b or relief valve 62b.

The tension in the web 15 is controlled by a brake 16 which is operatively connected to a brake control circuit 17 which also operates as described above.

FIG. 4

In the embodiment of the invention shown in FIG. 4 the web roll 12 is shown mounted on a spindle with the web threaded around a guide roll 22, the dancer roll 18 and a guide roll 24. The tension in the loop 23 formed in the web 15 is controlled by means of a dancer roll 18 which is supported for rotation on a pair of arms as described in H6. 1. Since the dancer roll 18 acts in a downward direction in applying tensions to the web 15, dancer excursions must be controlled in both directions, i.e., up and down;

Dancer, excursions in the downward direction are controlled by a dancer load circuit 50a which is connected to respond to pressure pulses from a piston and cylinder assembly 520. The piston and cylinder assembly 52a includes a piston 64a having a connecting rod 53a connected to the pivot arm 20 for the dancer roll 18. The dancer load circuit 50a includes an accumulator 54a and a differential pressure relief valve 62a which are connected to the piston and cylinder assembly 52a by pressure lines 660 and 69a. The load circuit 500 is identical to the circuit shown in FIG. 1 and operates as described above.

Dancer excursions in the vertically upward direction are controlled by means of a dancer load circuit 50b which is connected to respond to pressure pulses from a piston and cylinder assembly 52b. The piston and cylinder assembly 52b includes a piston 64b connected to the arm 20 by a piston rod 53b. The assembly 52b is connected to the load circuit 50b by a line 66b.

However, the connection is made to the opposite side of assembly 52b in order to control upward movement of the dancer roll 18. The dancer load circuit 50b includes an accumulator 54b and a differential pressure relief valve 62b as described above.

The brake v16 for spindle 10 is controlled by means of a brake control circuit 17 as described above.

I claim:

1. An apparatus for maintaining unifonn tension in a web being wound onto or unwound from a spool mounted for rotation on a spindle, said apparatus comprising:

. a signal responsive brake for retarding the rotation of the spindle to regulate the tension of the web,

a shiftably mounted dancer roll positioned to respond to a change in the length of the web,

a pneumatic pressure brake control circuit connected to said dancer roll for controlling the operating pressure of said brake, said brake control circuit including a pressure regulating valve and a spool diameter compensating cam positioned to control said valve and connected to respond to the movement of said dancer roll, said compensating cam including a cam surface having an angle of rise (1) equal to K, (log D log d) where D equals the diameter of a full spool, d is the instantaneous, diameter of said spool and K, is a constant of proportionality,

and a pneumatic dancer load circuit operatively connected to said dancer roll to maintain a predetermined tension on said web.

2. An apparatus for maintaining uniform tension in a web being wound onto or unwound from a spool mounted for rotation on a spindle, said apparatus com- 5 prising:

a signal responsive brake for retarding the rotation of the spindle to regulate the tension of the web,

a shiftably mounted dancer roll positioned to respond to a change in the length of the web,

a pneumatic pressure brake control circuit connected to said dancer roll for controlling the operating pressure of said brake, said brake control circuit including a first pressure regulating valve connected to respond to the movements of said dancer roll, an infinitely variable ratio relay connected to said first pressure regulating valve and to said brake for controlling the pressure on said brake in response to changes in the diameter of said roll, and a pneumatic dancer load circuit operatively connected to said dancer roll to maintain a predetermined tension on said web.

3. An apparatus for maintaining a uniform tension in a web being wound onto or unwound from a spool mounted for rotation on a spindle, said apparatus comprising:

a signal responsive brake for retarding the rotation of the spindle to regulate the tension of the web,

a shiftably mounted dancer roll positioned to respond to a change in the length of the web,

a pneumatic pressure brake control circuit connected to said dancer roll for controlling the oeprating pressure of said brake, and a pneumatic dancer load circuit operatively connected to said dancer roll to maintain a predetermined tension on said web, said pneumatic dancer load circuit including a pneumatic pressure line, an air cylinder connected to respond to movements of said dancer roll and to said pressure line and a rate compensated differential pressure relief valve connected to said pressure line for damping high amplitude dancer excursions.

4. An apparatus according to claim 3 including a second pressure line, a second air cylinder connected to respond to downward movements of said dancer roll and to said second pressure line, and a second rate compensated differential pressure relief valve connected to said second pressure line for controlling high amplitude downward dancer excursions.

5. An apparatus according to claim 3 wherein said dancer load circuit includes a check valve in the pneumatic pressure line, said differential pressure relief valve including a line pressure chamber connected to the pneumatic line on one side of said check valve and a relief chamber connected to the pneumatic line on the other side of said check valve.

6. An apparatus according to claim 5 including a needle valve connected to said pneumatic pressure line across said check valve to balance minute variations in pressure in said relief valve to system pressure.

7. An apparatus for maintaining uniform tension in a web being wound onto or unwound from a spool mounted for rotation on a spindle, said apparatus comprising:

a signal responsive brake for retarding the rotation of the spindle to regulate the tension of the web,

a shiftably mounted dancer roll positioned to respond to a change in the length of the web,

a pneumatic pressure brake control circuit connected to said dancer roll for controlling the operating pressure of said brake, and a pneumatic dancer load circuit operatively connected to said dancer roll to maintain a predetermined tension on said web, said pneumatic dancer load circuit including a pneumatic pressure line, an air cylinder connected to said dancer roll and to said pneumatic pressure line,

an accumulator connected to said pressure line for controlling high frequency low amplitude dancer movements and a rate compensated differential pressure relief valve connected to said pressure line for controlling high amplitude low frequency dancer excursions.

8. An apparatus according to claim 7 wherein said dancer load circuit includes a second pressure line, a second air cylinder connected to said dancer roll and to said second pressure line, a second accumulator connected to said second pressure line and a second rate compensated differential pressure relief valve connected to said second pressure line.

9. An apparatus according to claim 7 wherein said differential pressure relief valve includes,

a housing,

a valve port in said housing for venting said housing to atmosphere,

a piston rod in said housing for closing said port,

means for subjecting said piston rod to system pressure to close said port,

and means for subjecting said piston to pressure surges in response to dancer excursions, said piston rod moving in response to a predetermined pressure in excess of system pressure to open said valve port and vent said housing to atmosphere.

10. An apparatus according to claim 7 wherein said dancer load circuit includes a check valve in the pneumatic pressure line, said differential pressure relief valve including a line pressure chamber connected to the pneumatic line on one side of said check valve and a relief chamber connected to the pneumatic line on the other side of said check valve.

11. An apparatus according to claim 10 including a needle valve connected to said pneumatic pressure line across said check valve to balance minute variations in pressure in said relief valve to system pressure.

12. A pneumatic web tension control system for maintaining a uniform tension in a web being wound onto or unwound from a spool mounted on a spindle,

a signal responsive brake for retarding the speed of rotation of the spindle to regulate the tension of the web, and

a dancer roll for maintaining the tension of said web,

said system comprising means connected to respond to dancer roll excursions for producing pressure pulses,

and a pneumatic dancer load circuit connected to said pulse means, said load circuit including a pneumatic pressure line connected to said pulse means, and a differential pressure relief valve connected to said pressure line for venting high pressure pulses to atmosphere to damp high amplitude pulses.

13. The system according to claim 12 wherein said differential pressure relief valve includes a housing, a valve port in said housing for venting said housing to atmosphere, a piston rod in said housing having a valve for closing said port, means for subjecting said piston rod to system pressure to close said port, and means for subjecting said piston to pressure surges in response to dancer excursions, said piston rod moving in response to a predetermined pressure in excess of system pressure to open said valve port and vent said housing to atmosphere.

14. The system according to claim 12 including a brake pressure control circuit connected to said brake,

said brake pressure control circuit including a compensating cam connected to respond to movements of said dancer roll to compensate for variation in roll diameter,

and a pressure regulating valve positioned to respond to movements of said cam.

15. The system according to claim 14 wherein said cam includes a cam surface having an angle d) equal to K, (log D log d) where D equals the diameter of a full spool, d is the instantaneous diameter of said spool and K is a constant of proportionality.

16. The system according to claim 12 including a check valve in said pressure line, said differential pressure relief valve including a line pressure chamber connected to said pressure line on the input side of said check valve and a pressure relief chamber connected to said pneumatic line on the other side of sad check valve.

17. The system according to claim 12 including an accumulator connected to said pressure line for suppressing low amplitude high frequency pulses in said pulsing means.

18. In an apparatus for maintaining uniform tension in a web being wound onto or unwound from a spool mounted for rotation on a spindle, said apparatus including a dancer roll, a signal responsive brake posi- 10 an angle (1) equal to K (log D log d) where D equals the diameter of a full spool, d is the instantaneous diameter of said spool and K is a constant of proportionaling cam in said brake control circuit connected to re- 5 spond to movement of the dancer roll, said cam having 

1. An apparatus for maintaining uniform tension in a web being wound onto or unwound from a spool mounted for rotation on a spindle, said apparatus comprising: a signal responsive brake for retarding the rotation of the spindle to regulate the tension of the web, a shiftably mounted dancer roll positioned to respond to a change in the length of the web, a pneumatic pressure brake control circuit connected to said dancer roll for controlling the operating pressure of said brake, said brake control circuit including a pressure regulating valve and a spool diameter compensating cam positioned to control said valve and connected to respond to the movement of said dancer roll, said compensating cam including a cam surface having an angle of rise phi equal to K1 (log D - log d) where D equals the diameter of a full spool, d is the instantaneous diameter of said spool and K1 is a constant of proportionality, and a pneumatic dancer load circuit operatively connected to said dancer roll to maintain a predetermined tension on said web.
 2. An apparatus for maintaining uniform tension in a web being wound onto or unwound from a spool mounted for rotation on a spindle, said apparatus comprising: a signal responsive brake for retarding the rotation of the spindle to regulate the tension of the web, a shiftably mounted dancer roll positioned to respond to a change in the length of the web, a pneumatic pressure brake control circuit connected to said dancer roll for controlling the operating pressure of said brake, said brake control circuit including a first pressure regulating valve connected tO respond to the movements of said dancer roll, an infinitely variable ratio relay connected to said first pressure regulating valve and to said brake for controlling the pressure on said brake in response to changes in the diameter of said roll, and a pneumatic dancer load circuit operatively connected to said dancer roll to maintain a predetermined tension on said web.
 3. An apparatus for maintaining a uniform tension in a web being wound onto or unwound from a spool mounted for rotation on a spindle, said apparatus comprising: a signal responsive brake for retarding the rotation of the spindle to regulate the tension of the web, a shiftably mounted dancer roll positioned to respond to a change in the length of the web, a pneumatic pressure brake control circuit connected to said dancer roll for controlling the oeprating pressure of said brake, and a pneumatic dancer load circuit operatively connected to said dancer roll to maintain a predetermined tension on said web, said pneumatic dancer load circuit including a pneumatic pressure line, an air cylinder connected to respond to movements of said dancer roll and to said pressure line and a rate compensated differential pressure relief valve connected to said pressure line for damping high amplitude dancer excursions.
 4. An apparatus according to claim 3 including a second pressure line, a second air cylinder connected to respond to downward movements of said dancer roll and to said second pressure line, and a second rate compensated differential pressure relief valve connected to said second pressure line for controlling high amplitude downward dancer excursions.
 5. An apparatus according to claim 3 wherein said dancer load circuit includes a check valve in the pneumatic pressure line, said differential pressure relief valve including a line pressure chamber connected to the pneumatic line on one side of said check valve and a relief chamber connected to the pneumatic line on the other side of said check valve.
 6. An apparatus according to claim 5 including a needle valve connected to said pneumatic pressure line across said check valve to balance minute variations in pressure in said relief valve to system pressure.
 7. An apparatus for maintaining uniform tension in a web being wound onto or unwound from a spool mounted for rotation on a spindle, said apparatus comprising: a signal responsive brake for retarding the rotation of the spindle to regulate the tension of the web, a shiftably mounted dancer roll positioned to respond to a change in the length of the web, a pneumatic pressure brake control circuit connected to said dancer roll for controlling the operating pressure of said brake, and a pneumatic dancer load circuit operatively connected to said dancer roll to maintain a predetermined tension on said web, said pneumatic dancer load circuit including a pneumatic pressure line, an air cylinder connected to said dancer roll and to said pneumatic pressure line, an accumulator connected to said pressure line for controlling high frequency low amplitude dancer movements and a rate compensated differential pressure relief valve connected to said pressure line for controlling high amplitude low frequency dancer excursions.
 8. An apparatus according to claim 7 wherein said dancer load circuit includes a second pressure line, a second air cylinder connected to said dancer roll and to said second pressure line, a second accumulator connected to said second pressure line and a second rate compensated differential pressure relief valve connected to said second pressure line.
 9. An apparatus according to claim 7 wherein said differential pressure relief valve includes, a housing, a valve port in said housing for venting said housing to atmosphere, a piston rod in said housing for closing said port, means for subjecting said piston rod to system pressure to close said port, and means for subjecting said piston to pressure surges in response to dancEr excursions, said piston rod moving in response to a predetermined pressure in excess of system pressure to open said valve port and vent said housing to atmosphere.
 10. An apparatus according to claim 7 wherein said dancer load circuit includes a check valve in the pneumatic pressure line, said differential pressure relief valve including a line pressure chamber connected to the pneumatic line on one side of said check valve and a relief chamber connected to the pneumatic line on the other side of said check valve.
 11. An apparatus according to claim 10 including a needle valve connected to said pneumatic pressure line across said check valve to balance minute variations in pressure in said relief valve to system pressure.
 12. A pneumatic web tension control system for maintaining a uniform tension in a web being wound onto or unwound from a spool mounted on a spindle, a signal responsive brake for retarding the speed of rotation of the spindle to regulate the tension of the web, and a dancer roll for maintaining the tension of said web, said system comprising means connected to respond to dancer roll excursions for producing pressure pulses, and a pneumatic dancer load circuit connected to said pulse means, said load circuit including a pneumatic pressure line connected to said pulse means, and a differential pressure relief valve connected to said pressure line for venting high pressure pulses to atmosphere to damp high amplitude pulses.
 13. The system according to claim 12 wherein said differential pressure relief valve includes a housing, a valve port in said housing for venting said housing to atmosphere, a piston rod in said housing having a valve for closing said port, means for subjecting said piston rod to system pressure to close said port, and means for subjecting said piston to pressure surges in response to dancer excursions, said piston rod moving in response to a predetermined pressure in excess of system pressure to open said valve port and vent said housing to atmosphere.
 14. The system according to claim 12 including a brake pressure control circuit connected to said brake, said brake pressure control circuit including a compensating cam connected to respond to movements of said dancer roll to compensate for variation in roll diameter, and a pressure regulating valve positioned to respond to movements of said cam.
 15. The system according to claim 14 wherein said cam includes a cam surface having an angle phi equal to K1 (log D - log d) where D equals the diameter of a full spool, d is the instantaneous diameter of said spool and K1 is a constant of proportionality.
 16. The system according to claim 12 including a check valve in said pressure line, said differential pressure relief valve including a line pressure chamber connected to said pressure line on the input side of said check valve and a pressure relief chamber connected to said pneumatic line on the other side of sad check valve.
 17. The system according to claim 12 including an accumulator connected to said pressure line for suppressing low amplitude high frequency pulses in said pulsing means.
 18. In an apparatus for maintaining uniform tension in a web being wound onto or unwound from a spool mounted for rotation on a spindle, said apparatus including a dancer roll, a signal responsive brake positioned to retard the speed of rotation of the spindle to regulate the tension on the web and a pneumatic pressure brake control circuit controlling said brake, the improvement comprising a spool diameter compensating cam in said brake control circuit connected to respond to movement of the dancer roll, said cam having an angle phi equal to K1 (log D -log d) where D equals the diameter of a full spool, d is the instantaneous diameter of said spool and K1 is a constant of proportionality. 